Due to its excellent mechanical strength and cushioning properties, a polypropylene resin foam is widely used as a packaging material, a building material, a heat shield material and the like. However, since polypropylene has a high crystallinity and a low melt viscosity and is difficult to cross-link, it has hereto been quite difficult to obtain a highly expanded product from the polypropylene.
U.S. Pat. No. 5,527,573 (issued Jun. 18, 1996) discloses extruded closed-cell polypropylene resin foam and several methods for producing said foam. The foam of the US patent is in a plank form and has a minimal cross-sectional area of about 5×2.54 square centimeters, a minimal thickness of 12.7 millimeters and a density of about 5 pounds per a cubic foot. The form and properties make it difficult to mold the foam into desired shaped articles. U.S. Pat. No. 6,051,617 (issued Apr. 19, 2000) discloses a foamed polypropylene resin particle useful for molding a foamed, molded article and a method of preparing the same. However, the foamed polypropylene resin particle is prepared by grafting a vinyl comonomer to polypropylene resin particles to form the modified copolymer resin particles and foaming the modified copolymer resin particles. U.S. Pat. No. 6,077,875 (issued Jun. 20, 2000) discloses foamed and expanded beads of a polypropylene resin for molding prepared from a non-crosslinked propylene-ethylene random copolymer. The foamed beads of the US patent has an open cell content of at most 40%, most preferably 25% and has a melting point of at least 141° C.